What? Subwoofer placement is NOT that important. The frequencies from a subwoofer are nondirectional (unlike frequencies from mids & tweets which are more unidirectional which is why the best imaging is to have them pointed right at the listening position).
I am not 100% sure if car audio is like home audio but yes sub placement is very important. In home audio if the sub is not placed in the correct location you can create a NULL. I was at a Sencore (they build calibrating gear for audio and video) training class and they created on of these. You could take two steps in a certain direction and BAM not a damn thing. then two steps back and you could feel the base or a few steps forward.
But like I said not sure if this will happen in car audio. I do not see why it would not!!